1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a transistor operating at high frequencies.
2. Description of the Background Art
In recent years, electronic circuits operating at high frequencies have been in dramatically increasing demand because of the worldwide expansion of the market for mobile communication equipment typified by portable telephones and the like and the rapid proliferation of satellite communication services. Additionally, with the increase in the amount of transmitted information, there has been a need for communication at higher frequencies. To meet such requirements, MMICs (Monolithic Microwave Integrated Circuits) operating at high frequencies in the microwave region and for the formation on compound semiconductor substrates have been developed actively.
Of the MMICs, particularly important devices are transistors, which in turn operate at very high speeds because the compound semiconductor has a high electron mobility. One of such transistors is known as a HEMT (High Electron Mobility Transistor).
A structure of a conventional HEMT will be described. An electron transit layer is formed on a compound semiconductor substrate, and an electron supply layer is formed on the electron transit layer. First to third heavily doped layers are partially formed on the electron supply layer. A first source electrode is formed on the first heavily doped layer, and a drain electrode is formed on the second heavily doped layer. A second source electrode is formed on the third heavily doped layer. A first gate electrode having an overhanging shape is formed on a portion of the electron supply layer which lies between the first heavily doped layer and the second heavily doped layer. A second gate electrode having an overhanging shape is formed on a portion of the electron supply layer which lies between the second heavily doped layer and the third heavily doped layer. A resin having a relative dielectric constant ranging from about 4 to about 5 is formed to cover the first and second gate electrodes. A metal interconnect line is formed on the resin. The metal interconnect line is connected to the first and second source electrodes through first and second contact holes formed in the resin.
A gate voltage applied to the first and second gate electrodes is used to adjust the amount of electrons supplied from the electron supply layer to the electron transit layer, thereby controlling the amount of current flowing between the source electrodes and the drain electrode.
Techniques for forming an air space around the gate electrode of a HEMT are disclosed in Japanese Patent Application Laid-Open No. 2001-118859, Japanese Patent Application Laid-Open No. 6-140440 (1994), and Japanese Patent Application Laid-Open No. 2002-299443.
In the above-mentioned HEMT structure, there are intrinsic inevitable capacitors between the first and second heavily doped layers and the first gate electrode and between the second and third heavily doped layers and the second gate electrode. In the conventional HEMT, these capacitors have relatively large parasitic capacitances because the resin having the relative dielectric constant ranging from about 4 to about 5 covers the first and second gate electrodes. As a result, the conventional HEMT presents a problem in that the parasitic capacitances result in the reduction in electrical characteristics of the device in a high frequency range.